This invention relates to an amplitude modulation circuit for a transmitter in which overmodulation, as well as production of unnecessary spurious signals, is suppressed by means of an automatic level control (ALC) circuit.
In general, modulation circuits for transmitters are required to be capable of modulating a carrier to a sufficient degree with minor distortion in order to improve the signal to noise ratio (S/N ratio) and the articulation of call. Above all, when used with a radio transmitter, the modulation circuit must be minimally subject to production of spurious signals which may interfere with other frequency bands. As a modulation circuit in accordance with these requirements there has conventionally been used the following composition. That is, an input signal for modulation or audio-frequency signal (AF signal) for modulating the modulation circuit is supplied to a modulation unit through a limiter circuit or compression circuit. The limiter circuit or compression circuit has a function to prevent automatically overmodulation exceeding a modulation degree of 100%. That is, these circuits are so designed as to augment the mean transmission output by restraining overmodulation while securing sufficiently high mean modulation degree by supplying a high-level input signal for modulation to the modulation unit, thereby improving the S/N ratio.
The limiter circuit out of the aforesaid two circuits, however, has a defect to cause distortion of the modulation input signal. On the other hand, the compression circuit will hardly distort the input signal for modulation. Transmitters (transceivers) employing such compression circuit have already been proposed. Where a sufficiently high modulation degree was required, these transmitters could not satisfactorily restrain the production of spurious signals despite the use of such compression circuit, let alone the use of the limiter circuit.
The reason may be explained as follows. That is, the modulation circuit as an analog circuit is necessarily subject to non-linearity. Therefore, when the modulation input signal is supplied to the modulation circuit, the modulation circuit produces a harmonic distortion attributable to the non-linearity. The modulated wave or transmitted radio wave involving the harmonic distortion includes spurious components attributable to the harmonic distortion, besides the frequency components of the carrier. That is, the level of spurious signals is chiefly attributable to the non-linearity of the modulation circuit used with the transmitter. Generally, the higher the modulation degree as well as the frequency of the modulation signal, the more remarkable the non-linearity is. Therefore, in the transmitter provided with the prior art modulation circuit, the modulation degree with respect to the whole range of the frequency band of the AF signal used as the modulation input signal is restricted to a somewhat lower level in order to restrain the production of spurious signals. Thus, the prior art transmitter is so devised that any spurious signals exceeding a level prescribed by the specifications of the transmitter will not be caused outside the occupied band thereof if a modulation input signal having many higher range frequency components within the audio-frequency band is supplied to the modulation circuit. Further, in the transmitter including the conventional modulation circuit, a voltage controlled oscillator (VCO) in a frequency synthesizer circuit to produce the carrier and a radio-frequency amplifying circuit to amplify the carrier are securely shielded in order to avoid the modulation distortion attributable to unfavorable connections among the circuits within the transmitter.
The defects of the transmitter employing the prior art modulation circuit, as described above, may be summarized as follows. Since the production of spurious signals must be restrained, the performance primarily required for a transmitter, i.e. high mean modulation degree, cannot be secured. Moreover, in order to restrain the production of spurious signals, the transmitter requires well-selected, high-performance circuit elements as well as a number of components for shielding. Furthermore, in order to make the most of the performance of the transmitter within the range of modulation degree limited so as to bring the production of spurious signals in compliance with the specifications, many adjusting processes will be required for the transmitter. Accordingly, such type of transmitter cannot help being highly expensive.